ABSTRACT Melanoma is a major world health problem with an incidence rate that is rising rapidly. Within the past several years, there has been tremendous progress in novel melanoma therapies ? particularly with regard to immunotherapy as highlighted by the FDA approval of ipilimumab (anti-CTLA-4 antibody) in 2011 and pembrolizumab and nivolumab (anti-PD-1 antibodies) for melanoma in 2014 and 2015. Furthermore, when CTLA-4 and PD-1 blockade are combined, response rates are significantly increased, leading to the FDA approval of this regimen (ipilimumab + nivolumab) in 2015 ? however, toxicity is admittedly high. Despite these advances, there are still a significant proportion of patients who do not achieve clinical response to these agents. Therefore, there is tremendous need to identify biomarkers that may predict response or resistance to immune checkpoint blockade ? either as monotherapy or in combination ? and to identify actionable strategies that will enhance the effectiveness of these potent therapies. Our group has been actively engaged in efforts to better understand responses to immune checkpoint blockade therapies. In these studies, we performed deep immune and molecular profiling in a cohort of patients on immune checkpoint blockade, and recently demonstrated that an immune infiltrate in early on-treatment tumor biopsies is highly predictive of response. However it remains unclear what contributes to enhanced responses, and there is a critical need to identify actionable strategies to improve responses to therapy in all patients. There is a growing appreciation of the role of the gut microbiome in shaping immune responses in health and disease, and pre-clinical evidence that bacteria present within the gut may modulate differential responses to immune checkpoint blockade in melanoma, though this concept has not been studied in patients. This represents a significant knowledge gap, and insights gained could lead to therapeutic strategies to enhance responses to immune checkpoint blockade in melanoma. We have begun to address this knowledge gap in clinical samples from melanoma patients treated with immune checkpoint blockade, and are also studying this in a murine melanoma model. We have preliminary evidence suggesting that differential signatures exist in responders versus non-responders to therapy, and have insight into mechanisms via immune profiling and metabolomic studies. We have also generated data in a murine melanoma model, demonstrating differential tumor growth in C57BL/6 mice with identical genomes but differing gut microbiomes. We will now build on these studies through this proposal to explore the role of the microbiome in shaping anti-tumor immune responses and clinical responses to immune checkpoint blockade in melanoma.